Periodically corrugated surfaces (gratings) are widely used in many different optoelectronic devices. For example, a surface grating structure can be used to provide a feedback path for distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers. As an input-output coupler for selectively exciting the modes of an optical waveguide, gratings have advantages over other coupling techniques in being an integral part of the waveguide, mechanically simple, and capable of coupling into waveguides fabricated from high index materials. Additional applications include integrated narrowband filters, light deflectors, and phase matching elements.
A conventional method of forming the grating structure is referred to as a "holographic" method, in which a grating relief pattern is produced by interferometric exposure and development of photoresist on the wafer surface. The grating is then transferred to the substrate by ion-beam milling or chemical etching. Various techniques exist to generate the required interference pattern, including splitting a signal into two beams (spatially), then redirecting the two beams to the wafer surface to form the desired interference pattern. In an alternative arrangement, referred to herein as a "corner cube", the original exposure beam is directed at a mirror surface disposed at 90.degree. with respect to the wafer. The reflections from the mirror, in combination with the beam directly impinging the wafer, will form an interference pattern on the wafer, where the angle of incidence of the exposure beam on the mirror will determine the periodicity of the grating formed on the wafer.
In many situations it is necessary to control the grating structure and periodicity as carefully as possible. For example, current DFB lasers utilize a grating a structure with dimensions on the order of 0.1 .mu.m lines and spaces. Small angular errors when manufacturing the grating by UV exposure can have drastic effects on the performance and manufacturing of the DFB lasers. In the "corner cube" exposure arrangement, such angular errors have been attributed to misalignment of the wafer with the reflective surface, thus varying the grating exposed on the wafer surface. In particular, a misalignment may result in the grating varying in periodicity across the surface of the wafer. The repeatability of the wafer-to-mirror alignment is also problematic, resulting in the gratings being slightly different on each wafer as a number of wafers are processed in succession.